U.S. Pat. No. 6,734,643 discloses a lighting system comprising a feed circuit part, a lamp unit which contains a gas discharge lamp and a starter circuit. The starter circuit is divided in a starter drive part, which is contained in the feed circuit part, and a high voltage generating part, which is contained in the lamp unit. The feed circuit part further comprises a feed circuit for supplying an alternating supply voltage to the lamp unit, and a starter drive part. The lamp unit comprises a lamp and a high voltage generating part. Inner electrodes of the lamp are connected via two terminals of the lamp unit and via two terminals of the feed circuit part to the supply source. An outer auxiliary electrode of the lamp and an inner electrode of the lamp are connected to the high voltage generating part of the lamp unit. The high voltage generating part is connected via two terminals of the lamp unit, of which one terminal connected also to an inner electrode, via terminals of the feed circuit part to the starter drive part of the feed circuit part. The starter drive part is supplied by the alternating supply voltage. The starter drive part comprises an electronic switch and a capacitor to alternately charge and discharge said capacitor by which a transformer of the high voltage generating part induces a sequence of high voltage pulses, which are supplied to the outer electrode of the lamp. Said pulses may discharge a portion of gas contained in the lamp between said outer electrode and one of the inner electrodes. This may cause a discharge of between the inner electrodes, that is to ignition of the lamp. As a result the alternating supply voltage supplied to the lamp will decrease. As a result the electronic switch of the starter drive part will fail to conduct anymore, so that the supply of high voltage pulses to the outer electrode of the lamp terminates. The electronic switch may comprise a thyristor or a SIDAC, which means that the starter circuit, and as a consequence the disclosed system as a whole, operates at a relatively low frequency. A frequency of less than 500 kHz is considered low in this description.
A disadvantage of said prior art is that it requires quite some hardware to provide the starter circuit, such as a high voltage transformer and an electronic switch. Another disadvantage of said prior art is that it requires three conductors to connect the feed circuit part to the lamp unit, of which one conductor is used during less of a second for igniting the lamp only. As a result a dedicated, expensive cable is required to connect the feed circuit part to the lamp unit. A further disadvantage of the prior art is that a supply voltage for the lamp having a high frequency, for example above 2 MHz will be too expensive to be used in many cases. If the hardware of the starter circuit would be made suitable to operate with such a high frequency it would require an even more dedicated high frequency cable containing three conductors.
A gas discharge lamp of the above type may comprise start-promoting means as shown in U.S. Pat. No. 6,734,643 and as described in more detail in U.S. Pat. No. 6,563,267. Accordingly, for each electrode, the lamp may comprise a seal incorporating an electric conductor in the form of a foil which connects the electrode inside the main space or vessel of the lamp to a metal wire, which projects to the exterior from the seal. The seal has a cavity, which contains a gaseous constituent, for example mercury vapor. The cavity constitutes the start-promoting means as a source of UV radiation when applying an electric voltage across the cavity. The UV radiation source is referred to as UV enhancer.